Industrial process controllers are commonly tuned to control an output to meet a particular output specification. Where the features of the system function ideally, setting the control to the ideal specification then produces the specified output with no variation. Material handling systems do not work ideally for numerous reasons, and as a result there is frequently a statical distribution in the output from the system. Where there is no functional difference between the high side and the low side results of the statistical distribution, setting the control to meet the center of the statistical distribution is a correct choice.
Functional differences may exist between the high and low sides of the distribution. One side of the distribution is then acceptable, while the other is not. For example a chemical reaction may occur on the high side of the distribution that produces a pollutant thereby spoiling the output, or the material or energy investment may exceed what is necessary thereby wasting resources. In the cases where an important difference the process needs to be controlled so the distribution exists in the output distribution, is on the preferred side of the specification and then to be within a measure of closeness to the specification.
Where the distribution is known and has a fixed value, a simple solution is to manually position the set point to a level offset from the specification by an amount sufficient to locate the distribution center in the preferred zone, but close enough to the specification so the amount of output falling in the unacceptable zone is tolerable. Unfortunately, an output distribution is generally not known, and is likely to change in time. Further, the feature important to control may not be the center of the distribution, which may not be symmetric, but the amount of product, or the existence of any product occurring in the unacceptable zone.